1. Field of the Invention
The present invention relates to an image forming apparatus which suppresses degradation of an image caused by fluctuation in charged potential of a photosensitive member.
2. Description of the Related Art
Image forming apparatuses that use an electrophotographic process or an electrostatic recording process are in widespread use, and a certain level of quality is required for images formed by these image forming apparatuses. Here, as one factor that degrades image quality, density unevenness in a conveyance direction of a recording material (sub-scanning direction) can be pointed out, with this being caused by leaving a photosensitive member, which is a charged body, and a charging unit being pressed together for an extended period.
For example, in a case where the photosensitive member is charged by a charge roller, a discharge gap between the charge roller and the photosensitive member needs to be kept constant. Therefore, the surface of the charge roller is made smooth. However, in a case where a contact charging method is used, when the charge roller is left for an extended period in a state of being pressed against the photosensitive member, the charge roller may be deformed at the contact area with the photosensitive member (hereinafter, this deformation is referred to as a pressed mark, or simply a mark). This situation corresponds, for example, to a case in which a process cartridge having the charge roller is left unused for an extended period. For the charge roller having a mark, the discharge gap between the charge roller and the photosensitive member cannot be maintained constant. Accordingly, when charging of the photosensitive member is performed by the charge roller with a mark, fluctuation of the charged potential of the photosensitive member occurs when the mark of the charge roller passes a discharging area, and as a result density unevenness occurs in a rotation cycle of the charge roller.
Japanese Patent Laid-Open No. 2002-229306 proposes to suppress the density unevenness by controlling the amplitude of the fluctuation of a charging voltage that is applied to the charge roller to be no larger than 1%, when the mark of the charge roller passes the discharging area.
However, in order to control the amplitude of the fluctuation of the charging voltage to be no larger than 1%, a high-voltage capacitor is required to suppress the amplitude of the fluctuation of DC voltage, which causes an increase in costs. Furthermore, in the configuration disclosed in the Japanese Patent Laid-Open No. 2002-229306, although the density unevenness caused by the mark can be suppressed by increasing the capacitance of the high-voltage capacitor, the rise time of the charging output becomes longer. Accordingly, a difference in the charged potential of the photosensitive member occurs depending on the location of the charge roller, and the density unevenness caused by this difference in the charged potential may arise. Note that as another solution to resolve the problem of the mark, a configuration in which extended pressing does not physically occur by separating the photosensitive member and the charge roller can be considered. However, the mechanical configuration needs to be modified, which results in significant cost increase. As described above, density unevenness which is caused by fluctuation of the charged potential of a photosensitive member that occurs at a mark of the charge roller or the like needs to be suppressed.